Under normal conditions, a healthy human eye focuses on near and distant objects by contraction and relaxation of the ciliary muscle thereby contracting and releasing the tension on the zonules in the eye. The contraction of the ciliary muscle releases zonular tension (accomodative state) and allows the lens to alter to a more globular or spherical resting shape. The relaxation of the ciliary muscle increases tension on zonules and elastic forces in the eye tissue overcome the inherent lens elasticity and result in stretching the lens equator and flattening the lens curvature (un-accomodative state).
In certain instances, for example when age-related opacification of the lens (cataract) interferes with vision, the natural crystalline lens of the eye needs to be removed. Generally, the natural lens is replaced with an artificial one, for example, an intraocular lens (IOL). Unfortunately, conventional IOLs, even those that profess to be accommodative, may be unable to provide sufficient spatial displacement of the lens along the optical axis to provide an adequate amount of accommodation for near vision.
In conventional extracapsular cataract surgery, the crystalline lens matrix is removed by phacoemulsification through a curvilinear capsularhexis leaving intact the thin walls of the anterior and posterior capsules, together with zonular ligament connections to the ciliary body and ciliary muscles. An intraocular lens is then placed in the capsular bag, which collapses around the IOL.
Conventional single-optic accommodative intraocular lenses (AIOL) rely on the interaction of the ciliary muscle with the zonule and capsule to induce movement of the optic of the AIOL along its optical axis. Typically, the AIOL is secured within the capsular bag by two or more haptics that translate the radial stretching force exerted on the capsular bag by the zonules in an attempt to achieve the desired axial displacement of the optic.
However, during the post-implantation fibrotic healing process, the anterior capsule fuses with the posterior capsule to form a rigid capsular disc. Loss of elasticity of the capsular disc results and constrains the amount of movement that can be generated by the zonular force or elastic recoil of the intraocular lens and therefore, leads to a decrease in the amount of axial displacement of the lens that can be achieved.
Various lens systems have been designed to address this loss of accommodation. Passive-shift single-optic lenses, the only accommodative lens currently marketed, were designed to move forward under ciliary muscle contraction. Accommodation in these systems, however, remains limited by the loss of elasticity in the post-fibrotic capsule. Even the limited amount of accomodative amplitudes generated by these lenses immediately after surgery is lost within the first few weeks or month after surgery as capsular fibrosis ensures.
Accommodative lens designs with single or multiple optic lens assemblies have been disclosed, for example, in U.S. Pat. Nos. and U.S. application nos. 2009/0125106, 2005/0209692, 2007/0156236, 2009/0005866, 2007/0005136, 2009/0248154. Dual optic lenses retain the problem of capsular fibrosis and loss of amplitude/movement even though they are reported to provide a significant amount of accommodation. However, concerns about possible long-term formation of interlenticular opacification remain.
More recently, a lens systems that employs an active-shift mechanism using repulsive mini-magnets as a means of making accommodation partially independent of the zonules and mechanical properties of the capsular bag was disclosed (see U.S. Pat. Application Nos. 2009/0204210 and 2007/0118216. Still other methods of achieving accommodation include introduction of a polymerizable fluid with a desired refractive index into the capsular bag (lens refilling). Extensive investigation into the feasibility of these methods is still needed.
U.S. Publication No. 2009/0234449 discloses an intraocular lens comprising an accommodating element that is in contact with a substantial portion of the zonular region; the accommodating element is positioned relative to optical element and configured to cooperate with the ciliary muscle, the zonules and/or the vitreous pressure in the eye to effect a shape change to the optical element. According to the '449 publication, prior art multiple lens systems can be cumbersome and also require an axial displacement unachievable with a collapsed capsular bag and resulting ineffective accommodative mechanisms.
The need remains therefore, for an intraocular lens system and an effective mechanism for improving the accommodative capacity of an IOL following implantation. None of the current lens concepts take into account that the devitalized capsular bag after cataract surgery changes its physical properties from an elastic sphere to a contracted rigid disc.